Cost Per Action is the effective cost of a particular user action (filling out a form, purchasing an item, etc.) generated by an ad campaign..
The method of determining the cost per action for a campaign is:
Sum of Campaign Costs / Number of Actions Taken = Cost per Action
Cost Per Acquisition is the effective cost of acquiring a new customer or subscriber.
The method of determining the cost per acquisition for a campaign is:
Sum of Campaign Costs / Number of Customers Acquired = Cost per Acquisition